Automobile traffic increases each year in most countries. With this increased traffic, there is a corresponding rise in automobile accidents. Furthermore, increases in traffic continue to tax overcrowded road systems far beyond their designed capabilities. Increased loads, crumbling roadways and faster moving traffic all increase the risk of an accident. Nevertheless, in most situations, drivers are either unaware of or ambivalent to the fact that they are travelling too close to a vehicle in front of them to stop in case of a sudden stop. It is estimated that one half of the total cost of automobile accidents is a result of rear end collisions.
An efficient collision warning system can alert drivers to react and adjust their driving habits in order to avoid most collisions and save at least $25 billion annually in traffic accident costs, injuries and fatalities.
Many researchers have investigated the automotive collision warning field. However, the devices produced in past for this purpose suffer from several problems, such as high false alarms, real targets being ignored, target ambiguity in a dense environment and slow operation.
Radar is best known in a pulse form. A pulse of radio energy is emitted and the radar receiver listens for a reflected return pulse from a target. When this pulse is received, the time interval is noted so that the distance can be calculated. The ability of the radar to resolve distance is limited to the length of the pulse. Automotive radar requires a range resolution of approximately 3 meters. This would require the pulse to be less than 10 nanoseconds long, given the average space between automobile and potential obstacles. A system of this type would be very costly to implement in a millimeter wave transceiver. A number of devices have been proposed which utilize pulsed radar. These include Kubota et al., U.S. Pat. No. 3,787,845, issued Jan. 22, 1974 and Ban et al., U.S. Pat. No. 3,898,653, issued Aug. 5, 1975. Each of these devices shares this shortcomings of pulsed radar.
The high rate of false alarms, which is generally associated with these pulse radar devices, is primarily caused by the multitude of road features that do not present a danger to the vehicle but which may momentarily appear to be a dangerous object to a detection system. These include guard rails, signs, buildings, overpasses and vehicles in other lanes. These objects reflect and scatter radio waves. The amount of reflection and scattering that is produced by these objects cannot generally be predicted or modeled. For example, it is very difficult for small pulsed radar units to distinguish between a car and a sign or guard rail.
The size of previous radar units has also been a problem. The radar's ability to distinguish angular direction is primarily dependent on the size of the antenna as compared to the wavelength of the radio frequency used. A larger antenna has a narrower beam and can therefore distinguish objects with greater precision. The antenna can be made smaller, but this requires much higher frequencies to be used and high frequency devices are usually very expensive and difficult to produce.
Lastly, the time required by the radar system to acquire a target and determine its velocity and distance is critical to the effectiveness of the unit. The multitude of targets and the high relative speeds found in highway driving introduce a time constraint on the speed of the signal processor. In the past, these units have been slow to react and easily confused.
There remains, therefore, a need in the art for a compact and simplified radar system which can be utilized to accurately predict an optimum space cushion between a vehicle and potential obstacles.